Siti Salwa Hashim, Subhash Bhatia, Abdul Rahman Mohamed*) · A standard Taguchi L9 (3³) Orthogonal...

Science of Sintering, 48 (2016) 177-189 ________________________________________________________________________

_____________________________

*) Corresponding author: [email protected]

doi: 10.2298/SOS1602177H UDK 692.533.1; 622.785 Parameter Optimization for Sintering of La0.6Sr0.4Co0.2Fe0.8O3-6 (LSCF6428) Perovskite-structured Ceramics by Taguchi Method Siti Salwa Hashim, Subhash Bhatia, Abdul Rahman Mohamed*)School of Chemical Engineering, Universiti Sains Malaysia, 14300 Nibong Tebal, Penang, Malaysia Abstract: The influence of sintering temperature, sintering time and heating rate on the shrinkage rate, sintered density and porosity of La0.6Sr0.4Co0.2Fe0.8O3-ó (LSCF6428) perovskite-structured ceramics has been analyzed and the optimum sintering parameters have been determined by using Taguchi experimental design method. The analysis of variance (ANOVA) shows that the most significant parameter is sintering temperature, while the sintering time and heating rate are the least significant parameters. The optimum sintering parameter combination within the experimental ranges was found to be 1400°C, 20 h and 10°C/min. The shrinkage rate of 15.97%, the sintered density of 6.06 g/cm³ and the porosity of 0.15% were obtained using these co-optimized sintering parameters. The SEM micrographs confirm that sintering temperature has pronounced effect on the densification and porosity reduction of LSCF6428 flat membranes. Keywords: La0.6Sr0.4Co0.2Fe0.8O3-ó (LSCF6428); Sintering; Taguchi method; Shrinkage rate; Sintered Density; Porosity.

1. Introduction

Dense La0.6Sr0.4Co0.2Fe0.8O3-ó (LSCF6428) perovskite-structured mixed ionic-electronic conducting (MIEC) ceramic membrane has attracted considerable attention due to its oxygen ion and electronic conductivity. It has excellent mechanical and chemical stability that finds its application in the oxygen separation membranes and membrane reactors at high temperatures. Sintering is one of the processes in the ceramic processing to produce a better membrane with desired structures and properties particularly in the field of oxygen separation from air [1]. High density of the sintered membrane is desirable to avoid the presence of isolated and enclosed pores after sintering that could result to a drop in the oxygen flux. This happens due to the extended oxygen ion diffusion distance or the large resistance induced by the many recycles of surface reactions [2]. It has been reported previously that the densification and microstructural evolution with different sintering conditions are interrelated to the variation of the mixed conduction properties of perovskite-structured MIEC membranes [3-8]. The effect of sintering conditions on the microstructure and physical properties of LSCF6428 has been well studied [3, 5, 9]; however, to our knowledge, the use of design of experiment (DOE) via Taguchi method has not been systematically investigated. The Taguchi method involves reducing the variation in a process through robust design of experiments [10-

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S. S. Hashim et al. /Science of Sintering, 48 (2016) 177-189 ___________________________________________________________________________

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12]. The Taguchi experimental design involves using orthogonal arrays (OAs) to organize the parameters affecting the process and the levels at which they be varied [11, 12]. This method tests pairs of combinations by simultaneously studying the process parameters, instead of testing all possible combinations like the factorial design. This experimental plan allows the determination of factors that most affect product quality with a minimum amount of experimentation at reduced research and development costs [13, 14].

The objective of the present research is to study the important parameters that influence the sintering process of LSCF6428 as flat membrane material using Taguchi method. The effect of sintering parameters was monitored by the physical properties of the membranes. The three independent process parameters namely sintering temperature, sintering time and heating rate were chosen as the controlling parameters for sintering of LSCF6428 membrane. The three parameters chosen were varied at three different levels. This three factors three level system by means of the Taguchi method has led to only nine experiments to investigate the most significant factor that influences the sintering process.

2. Experimental Procedure

Commercially available La0.6Sr0.4Co0.2Fe0.8O3-ó (LSCF6428) powder with the surface area of 6.3 m2/g and particle size (d50) of 0.32 μm (purchased from fuelcellmaterials.com (FCM), Lewis Center, Ohio) was used as the raw material. The LSCF6428 powder was compressed into a flat membrane in the form of a pellet with a die pressed by hydraulic press (Spacec, UK) at pressure 3 ton with a holding time of 60 s. An amount of 1-2 g of powders produced 1-2 mm-thick and 20 mm diameter green pellet. The green pellets were sintered at different sintering temperatures (1000°C, 1200°C and 1400°C), sintering time (5 h, 10 h and 20 h) and heating rates (3°C/min, 5°C/min and 10°C/min) in high temperature chamber furnace (Carbolite RHF1600, UK) operating with air atmosphere. The sintering behavior of the sintered pellets was evaluated by measuring the shrinkage rate, sintered density and porosity, which represents the variation of the ceramic properties as a function of sintering temperature, sintering time and heating rate. The green and sintered pellets were carefully measured at several sections using Vernier caliper (Mitutoyo, Japan) and the mean value being used for determining the linear shrinkage rate, LS (%) by means of the following equation:

100⋅−

=Lg

LLgLS S (1)

where Lg and Ls are the mean diameters of green and sintered pellets, respectively. The bulk density and apparent porosity were measured using Archimedes principle

according to ASTM C373-88, which involved drying the pellets to constant mass (wο), boiling in distilled water for 5 h and soak for an additional 24 h at ambient temperature [15]. The suspended mass of each pellet in water (w₁) and the saturated mass (w₂) were determined.

The bulk density, B (g/cm³) and apparent porosity, ε (%) were calculated using the following formulas:

12

0

www

B−

= (2)

10012

02 ⋅−−

=wwww

ε (3)

S.S. Hashim et al./Science of Sintering, 48 (2016) 177-189 ___________________________________________________________________________

179

The morphological features of the sintered pellets were examined by scanning electron microscopy (SEM) (Zeiss Supra 35VP, USA).

3. Experimental Design

A standard Taguchi L9 (3³) Orthogonal Array (OA) comprising of nine tests, three parameters, three level) was chosen for this study. This format was chosen from a preliminary work that identified three parameters namely sintering temperature, sintering time and heating rate as important sintering variables that affected the sintering behavior of LSCF6428. The criteria being considered for choosing the three parameter levels was based to explore a maximum range of experimental variable. It was unnecessary to have uniformly spaced levels due to the counterbalance property of the OA [10]. It was previously reported that dense LSCF6428 membranes were achieved at a sintering temperature as low as 1000°C and as high as 1300°C [3, 5]; for this study, the three levels of sintering temperature were chosen as 1000°C, 1200°C and 1400°C. The three levels for sintering time and heating rate were selected according to the previous preliminary work with extended level values in order to observe their effects. The chosen parameters and their levels are shown in Tab. I. The test run was designated by replacing the level number 1, 2 and 3 of parameters A, B and C in L9 OA. The test run design is shown in Tab. II.

Tab. I Factors and factor level selected in the experiment.

Level Parameter Code 1 2 3

Sintering temperature (°C) A 1000 1200 1400 Sintering time (h) B 5 10 20 Heating rate (°C/min) C 3 5 10 Tab. II Experimental layout and factor distribution of L9 orthogonal array.

Parameters Test No.

A: Sintering Temperature (°C) B: Sintering Time (h)

C: Heating rate (°C/min)

1 1000 5 3 2 1000 10 5 3 1000 20 10 4 1200 5 5 5 1200 10 10 6 1200 20 3 7 1400 5 10 8 1400 10 3 9 1400 20 5


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4. Data Evaluation And Analyis 4.1. Evaluation Of Signal-To-Noise Ratio

The signal-to-noise ratio or the S/N ratio was calculated for each test run conducted to determine the effect of each variable has on the experimental results. The maximum shrinkage rate and sintered density was the objective function of the present study; therefore, the larger-the-better S/N ratio (SNLTB) was chosen. The following equation was used to determine the S/N ratio for the case of maximizing the shrinkage rate and sintered density [16]:

B

⎥⎥⎦

⎤

⎢⎢⎣

⎡⎟⎟⎠

⎞⎜⎜⎝

⎛−= ∑

=

n

i iynNS

12

11log10 (4)

In this study, the minimum apparent porosity was also the objective function; thus, the smaller-the-better S/N ratio (SNSTB) was chosen for this case using the following equation [16]:

⎥⎦

⎤⎢⎣

⎡−= ∑

=

n

iiynN

S1

21log10 (5)

Where y2i is the measured property and n corresponds to the number of measurements taken in one test runs. 4.2. Level average response analysis

All influences from the different levels of other parameters are counterbalanced when the level average response analysis is conducted at each level for each parameter. The effect of one parameter at one level on the experiment results can be observed independently from other parameters. In this study, each level for each parameter has three test runs. It can be seen in Tab. I that the level 1 of parameter A occurs in test run 1, 2 and 3 as all three levels of parameters B and C appear once in three test runs. The level 2 of parameter A occurs in test run 4, 5 and 6, in which all three levels of parameter B and C also appear once in these three runs. The similar occurrence can also be observed for the level 3 of parameter A. These occurrences are also applied for other levels of other parameters. All three levels of parameter A and C appear once in the three test runs of 1, 4 and 7 for level 1 of parameter B. Fig. 1 and 2 show the results of level average response analysis carried out by averaging the experimental results and the S/N ratio from three test runs, respectively.

1 2

4

6

8

10

12

14

16

3

Shrin

kage

Rat

e (%

)

Level of Design Parameter

Sintering Temperature Sintering Time Heating Rate

a)


181

1 2 3

3.8

4.0

4.2

4.4

4.6

4.8

5.0

5.2

5.4

5.6

5.8

Sint

ered

Den

sity

(g/c

m3 )



b)

1 2

0

5

10

15

20

25

30

3

Poro

sity

(%)



c) Fig. 1. Level average responses for parameters A, B, and C of the measured properties, (a)

shrinkage rate, (b) sintered density and (c) porosity of the LSCF6428 flat membranes.

1 212

16

20

24

3

S/N

Rat

io (d

B)



a)


182

1 2

12

13

14

15

3

S/N

Rat

io (d

B)



b)

1 2

0

10

20

30

3

S/N

Ratio

(dB)



c)

Fig. 2. Level average responses for parameters A, B, and C by S/N ratios of the measured properties, (a) shrinkage rate, (b) sintered density and (c) porosity of the LSCF6428 flat

membranes. 4.3. Analysis of Variance (ANOVA)

The effects or the contribution of each of the three sintering factors on the output characteristics can be obtained from the ANOVA using the S/N ratio responses that has been previously calculated in Section 4.1. This statistical treatment is commonly applied to the experiment results to determine the percent contribution of each factor.

The following equation has been used to calculate the overall mean in which all the variation (standard deviation):

∑=

=n

i iNS

nNS

1

1 (6)

The grand total sum of squares (GTSS) is given by:

∑=

⎟⎟⎠

⎞⎜⎜⎝

⎛=

n

i iNSGTSS

1

2

(7)

The GTSS consists of the sum of squares due to overall mean and the sum of squares due to variation around overall mean:

iationmean SSSSGTSS var+= (8) The sum of the squares due to overall mean is given by:


183

2

⎟⎟⎠

⎞⎜⎜⎝

⎛⋅=

NSnSSmean (9)

where n is the total number of total test runs. The sum of the squares due to variation around overall mean is given by:

2

1var

1 ∑=

⎟⎟⎠

⎞⎜⎜⎝

⎛−⋅=

n

i iiation N

SNS

nSS (10)

∑=

⎟⎟⎠

⎞⎜⎜⎝

⎛−=

n

i iiation N

SNSSS

1

2

var (11)

The SSvariation can be further decomposed into the sum of the squares of the variation induced by individual parameter effects around the overall mean. It is known that all the influences from the different levels of the other parameters are counterbalanced when the level average response analysis is performed at each level of each parameter. Therefore, the variation that is caused by each parameter at each level can be represented by the level average response using S/N ratio minus the overall mean. For example, the sum of the squares due to the variation around the overall mean for parameter A is given by:

2

33

2

22

2

11 ⎟⎟

⎠

⎞⎜⎜⎝

⎛−⋅+⎟⎟

⎠

⎞⎜⎜⎝

⎛−⋅+⎟⎟

⎠

⎞⎜⎜⎝

⎛−⋅=

NS

NSn

NS

NSn

NS

NSnSS

AA

AA

AAA (12)

where nAi is the number of tests conducted at level 1 of parameter A and S/NAi is the level average S/N parameter A at level i.

Finally, the percentage contribution of each parameter is given by: Percentage contribution of Parameter j = (SSparameter j/SSvariation) (13)

Fig. 3 shows the percentage contributions for parameters A, B, and C on the shrinkage rate, sintered density and porosity.

Sintering Temperature Sintering Time Heating Rate0

20

40

60

80

100

Con

tribu

tion

(%)

Shrinkage Rate Sintered Density Porosity

Fig. 3. Percentage contributions for parameters A, B, and C determined by ANOVA.


184

5. Results and Discussion

The influences of individual sintering parameters on the shrinkage rate, sintered density and porosity of LSCF6428 flat membranes can be seen in Fig. 1. From Fig. 1 (a), the shrinkage rate increases sharply as the sintering temperature increases from 1000°C to 1200°C with an increase of 241.2%. The shrinkage rate only increases 1.9% when the sintering temperature increases from 1200°C to 1400°C. Within the experimental ranges of 5-20 h and 3-10°C/min, the sintering time and heating rate have minor effect on the shrinkage rate. The decisive part is probably played by high sintering temperature that accelerates the material transport mechanism on the surface of the particles, resulting to the compact shrinkage during the intermediate stage of sintering. The slight increase of the shrinkage rate with the increase of the sintering time and heating rate can be ascribed to a prolonged time scale for mass transport and diffusion with the use of faster heating rate to reach the sintering temperature.

A convenient way to confirm the indications brought about by shrinkage rate measurements and to gain knowledge on the influence of sintering conditions is constituted by density measurements. It can be seen in Fig. 1 (b) that the sintered density also increases sharply with the increase of sintering temperature from 1000°C to 1200°C with an increase of 39.3% and slightly increases when the sintering temperature increases from 1200°C to 1400°C with an increase of 1.8%. However, it is found that the sintered density decreases by 12.1% with the increase of sintering time from 5 h to 20 h. The sintered density increases with the increase of heating rate from 3°C/min to 5°C/min but slightly decreases from 5°C/min to 10°C/min. Similar observations were obtained by other researchers on the significant effect of sintering temperature on the densification of LSCF6428 flat membranes [3, 5, 9]. However, it is impossible to make a direct comparison of their result because of the differences in powder preparation methods that leads to different particle size and the experimental design approach used in this study. Nevertheless, this would be particularly useful in view of the fact that the sintering temperature has an apparent effect on the sintered density of LSCF6428 flat membranes.

A further notable feature from Fig. 1 (c) is that regardless of the sintering time and heating rate, the rapid decrease in porosity by 97.9% on increasing the sintering temperature from 1000 to 1200°C and slight increases from 0.60% to 0.86% when the sintering temperature increases from 1200°C to 1400°C. It can be assumed that the principal controlling factor here is the temperature and that the timing and heating rate effects are not really detecTab. in this experiment. It has also been reported that the porosity reduction of La0.6Sr0.4-Co0.8Fe0.2O3-δ membrane sintered at higher temperature is more pronounced than at lower temperature [17]. The porosity is expected to decrease with increasing temperature as the trapped gases escape by diffusion through the matrix. This phenomenon is driven by the dissolution that occurs under the Laplacian excess bubble pressure, according to Henry’s Law [18]. The slight increase of the porosity at 1400°C can be well-linked to the so-called body bloating due to the presence of gas inside the closed pores that tends to expand the pores [19]. The level average response analysis by S/N ratio in Fig. 2 shows that the optimum sintering parameter combination for maximum shrinkage rate is found to be sintering temperature 1400°C, sintering time 20 h and heating rate 10°C/min. For maximum sintered density, the optimum sintering parameter combination is sintering temperature 1400°C, sintering time 3 h and heating rate 5°C/min while for minimum porosity is sintering temperature 1400°C, sintering time 20 h and heating rate 3°C/min.

The analysis of variance (ANOVA) shows that within the experimental level ranges, the most significant sintering parameter that influence the shrinkage rate, sintered density and porosity of LSCF6428 flat membranes is sintering temperature, which accounts for 99.19%, 83.99% and 97.74%, respectively. This is followed by sintering time, which accounts for 0.35%, 7.31% and 0.59%, respectively. Heating rate only accounts for 0.15%, 5.63% and


185

1.59% contribution of the shrinkage rate, sintered density and heating rate. These percentage contributions of each sintering parameter to the shrinkage rate, sintered density and porosity is only valid within the experimental parameter setting level ranges, which is the sintering temperature from 1000°C to 1400°C, the sintering time from 5 h to 20 h and the heating rate from 3°C/min to 10°C/min.

6. Conformation Test

The confirmation test is required to validate the Taguchi method. Fig. 1 reveals that for maximum shrinkage rate, the sintering temperature should be at level 3, the sintering time should be at level 3 and the heating rate should be at level 3 (A3B3C3). For maximum density and minimum porosity, the sintering temperature should be at level 3, the sintering time should be at level 1 and the heating rate should be at level 3 (A3B1C3). Two confirmation test runs were performed to validate the model generated using input sintering parameter values as: (1) A = 1400°C, B = 20 hours and C = 10°C/min and (2) A = 1400°C, B = 5 hours and C = 10°C/min. Three pellets were used for each confirmation test run and the average shrinkage rate, density and porosity were calculated. The results are tabulated in Tab. III. It can be seen that shrinkage rate, density and porosity are significantly improved at the optimized levels of first confirmation test run as compared to the second confirmation test run. The density has not been compromised at the optimized levels of first confirmation run indicating that the confirmation test was successful for maximum shrinkage rate and sintered density as well as minimum porosity when the optimized parameters are: sintering temperature = 1400°C, sintering time = 20 hours and heating rate = 10°C/min. The shrinkage rate of 15.97%, the sintered density of 6.06 g/cm³ and the porosity of 0.15% were obtained using these co-optimized sintering parameters.

Tab. III Confirmation test. Confirmation run no.

Sintering temperature

(°C)

Sintering time (h)

Heating rate

(°C/min)

Shrinkage rate (%)

Density

(g/cm³)

Porosity

(%) 1 1400 20 10 15.97 6.06 0.15 2 1400 5 10 15.87 5.79 0.65 7. Microstructural Evolution

It is evident from Fig. 3 that sintering temperature has significant influence on the shrinkage rate, sintered density and porosity of LSCF6428 flat membranes. SEM micrographs are presented in Fig.s 4 (a-c) in order to verify that the sintering temperature may play an important part in the physical properties and thus the microstructural evolution of LSCF6428 flat membranes. Fig.s 4 (a) and (b) show the SEM micrograph of the surface and cross-section of the sintered LSCF6428 flat surface using the same Taguchi confirmation test co-optimization value for sintering time and heating rate. Fig. 4 (a) shows the image of the flat membrane sintered at 1200°C while Fig. 4 (b) at 1000°C. Fig. 4 (c) shows the SEM micrograph of the surface and cross-section of the sintered LSCF6428 flat membrane subject to the operating conditions found from Taguchi methodology and confirmation test with A = 1400°C, B = 20 hours and C = 10°C/min.

The flat membrane sintered at 1000°C is porous as shown in Fig. 4 (a). The microstructure does not show surface relaxation at the grain boundary of the flat membrane, which may indicate that the sintering process is dominated by surface diffusion that leads to


186

particle coarsening. In this case, the surface diffusion is too rapid to redistribute the material that has been removed from the grain boundary region; therefore, the particle coarsening will dominate over the densification throughout the initial stage of sintering [20]. During this occurrence, the total surface area is reduced by the growth of larger particles at the expense of the shrinkage of smaller particles and their eventual disappearance [21]. Higher sintering temperature at 1200°C has led to grain relaxation, which makes the flat membrane dense and some of the polyhedral shaped pores are already isolated, as demonstrated in Fig. 4 (b).

The micrograph image in Fig. 4 (c) shows the flat membrane is denser and complete grain relaxation can be seen on the surface of the flat membrane and no connected porosity across the flat membrane surface. The majority of the pores are small and spherical. The flat membrane is not 100% densified at the sintering temperature of 1400°C because there are still isolated pores clearly visible along the cross-section area. Complete porosity elimination and thus complete densification at the end of sintering is difficult because the residual porosities cannot be removed through grain growth mechanism, attribuTab. to longer diffusion distance for mass transport to eliminate closed pores [22]. The microstructural image shown in Fig.s 4 (b and c) demonstrates that grain boundary and/or lattice diffusion may dominate over surface diffusion leading to densification; and the source of material deposited at the material deposited at the particle neck is grain boundary [20]. This observation suggests that the dominant sintering mechanism may change with temperature since the different diffusion coefficients have different temperature dependences. The micrograph images in Fig.s 4 (a-c) not only confirm that the densification and porosity reduction of LSCF6328 flat membranes depend on the sintering temperature, but also significantly improved at the optimized levels obtained using the Taguchi method.

(b)


187

(c)

Fig. 4. The SEM images of LSCF6428 flat membranes sintered at different temperatures for

20 hours with heating rate of 10°C/min (a) 1000°C; (b) 1200°C; (c) 1400°C.

8. Conclusions

The Taguchi’s experimental design methodology has been successfully applied for sintering parameter optimization of La0.6Sr0.4Co0.2Fe0.8O3-ó (LSCF6428) flat membranes. Increasing the sintering temperature parameter has resulted in the sharp increase of the shrinkage rate and sintered density as well as the decrease in the porosity. The increase of the shrinkage rate and sintered density and the reduction of the porosity are almost linear with the increase of the sintering time and heating rate throughout the parameter range. Within the experimental ranges, the most significant influencing parameter is sintering temperature, which accounts for 99.18%, 83.99% and 97.74% of the total effect on the increase of shrinkage rate and sintered density, and the reduction of porosity, respectively while the sintering time and heating rate are the least significant parameters. The SEM micrographs confirm that sintering temperature has pronounce effect on the densification and porosity reduction of LSCF6428 flat membranes.The optimum sintering parameter combination within the experimental ranges was found to be A3, B3, and C3 (1400°C, 20 h and 10°C/min). The shrinkage rate of 15.97%, the sintered density of 6.06 g/cm³ and the porosity of 0.15% were obtained using these co-optimized sintering parameters.

Acknowlegments

The authors gratefully acknowledge the research funding provided by the Universiti Sains Malaysia through Research University grants (RU 814088 and RU TEAM 854100).

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Садржај: Испитиван је утицај температуре синтеровања, времена синтеровања и бтзина загревања на скупљање, густину и порозност La0.6Sr0.4Co0.2Fe0.8O3-ó (LSCF6428) перовскитне структурне керамике и применом Тагучијеве експерименталне методе одређени су оптимални параметри синтеровања. Варијабилна анализа (ANOVA) показала је да је најзначајнији параметар температура синтеровања, док су друга два најмање значајна. Оптимални параметри синтеровања су 1400°C, 20 h и 10°C/min. Скупљање од 15.97%, густина након синтеровања 6.06 g/cm³ и порозност 0.15% су добијени након примене ових параметара. Микрографије указују на то да температура синтеровања има снажан утицај на згушњавање и редукцију порозности LSCF6428 равних мембрана. Кључне речи: La0.6Sr0.4Co0.2Fe0.8O3-ó (LSCF6428); синтеровање; Тагучијев метод; брзина скупљања; густина; порозност

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Siti Salwa Hashim, Subhash Bhatia, Abdul Rahman Mohamed*) · A standard Taguchi L9 (3³) Orthogonal...

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